1. Field of the Invention
The present invention relates to a color image processing system and a method thereof, more specifically, it relates to a color image processing system and a method and a program thereof, which relate to a color correction technology for a printer having a color gamut smaller than that of an output machine to be a target.
2. Description of the Related Art
In a color management for an image processing system utilizing various devices, as one of data which plays an important role, there is a color profile. The color profile describes the color reproduction characteristic of the device, and is used in color matching processing for causing the output color of the device to be a desired color. As one of the standards of a general color profile, a profile standardized by ICC (International Color Consortium) is well known, and used normally in color matching processing.
Recently, as an ink-jet system printer and an electrophotographic printer have spread, a color management technology has been popular. A user can obtain an output appropriate for the object of the user, by setting a profile depending on the application of the output and selecting an optimal matching intent (when matching of different color spaces is performed by shrinking and extending them one another, the methods for shrinking and extending them). In particular, in many cases, the electrophotographic printer is also used for a simulation of a printing machine. With regard to the electrophotographic printer, there is a printer in which various types of profiles, such as a CMYK system profile such as Japan Color or DIC, and a RGB system profile such as sRGB or a profile for a scanner, can be specified as a source profile.
There are various sizes and shapes for an output color gamut of a printer, depending on devices. If a color gamut of the source profile and the output color gamut differ from each other significantly, a problem will occur in a color to be output. For example, FIG. 5 is a diagram illustrating the color gamut when the minimum lightness value of the source profile is lower than that of the output color gamut. In the case, the minimum lightness value minL_src of the source profile is smaller than that of the output color gamut minL_dst. Accordingly, there occurs a problem in that all chromaticity points within a range of lightness values of minL_src to minL_dst are mapped on minL_dst, causing gradation collapse.
In order to solve such a problem, a method for mapping after virtually correcting the output color gamut, particularly, as in FIG. 16, a low lightness part can be considered. As one of the correction methods, a method disclosed in Japanese Patent Laid-Open No. 2006-081080 has been proposed. This is a method when the minimum lightness value L* possessed by the color gamut of an output device differs from the minimum lightness value L*′ possessed by the color gamut to be a target. In other words, it is a method for performing mapping after virtually extending only the low lightness region of one color gamut having a higher minimum lightness value, and matching the lightness value range with that of the other color gamut.
If such a method is used, the gradient characteristics near a black point when a certain source profile is set, can be compensated. However, as mentioned above, especially, in case of MFP (Multi Function Peripheral: multi-function peripheral equipment), various source profiles can be set. Therefore, the color gamut of each source profile is various, and the difference in the minimum lightness values which can be expressed is also large. Since only a fixed color conversion table can be stored in a destination (printer) profile, some source profiles to be set may cause a problem that the gradient characteristics of a dark part of an output image are poor.
As measures to solve such a problem, a method for changing the correction method of the output color gamut depending on an input image is proposed (refer to Japanese Patent Laid-Open No. 10-200773 (1998)). In Japanese Patent Laid-Open No. 10-200773 (1998), a method is proposed, which detects the maximum value and the minimum value of lightness component of an input image signal and, before performing interpolation calculation on the lightness component of the input image signal, performs compression processing depending on the maximum lightness value and the minimum lightness value.
However, in Japanese Patent Laid-Open No. 2006-081080, since lightness correction is performed using a weighting function only in lightness direction, it is not possible to address a case in which the shape of the output color gamut of the dark part is distorted. Moreover, since similar correction is performed also in the color reproduction region, the parts except for the dark parts may be corrected similarly, resulting in poor matching accuracy.
Moreover, in Japanese Patent Laid-Open No. 10-200773 (1998), a method is proposed which changes compression processing of Gamut (a color gamut of each of a monitor and a printer etc.) depending on the maximum/minimum lightness values of an input image signal, however, if processing is performed on every input image, long time will be necessary for processing. Moreover, in some images, since color difference should be paid greater attention than gradient characteristics, there is a problem in this method, that such cases cannot be addressed flexibly. Moreover, when the same source profile and the same image are input into a plurality of printers existing on a network as in FIG. 17, due to difference of the output color gamut (Gamut) of each of the printers, it is difficult to obtain output image giving the same impression.